The invention relates to a Balun assembly having a coaxial cable and, more particularly to a more reliable, easily manufactured and electrically repeatable Balun assembly. Balun assemblies have a coaxial cable to join with a radio frequency (xe2x80x9cRFxe2x80x9d) launch area on a circuit board.
A known Balun assembly is a three port device having one 50 ohm impedance port and two 25 ohm impedance ports. When used on a push-pull RF device with 25 ohm matching structures (two inputs and two outputs), a Balun at the input will be needed as a splitter, splitting the 50 ohms input to two 25 ohm outputs which then feed the RF device. If used at the output of the two 25 ohm matching structures, the Balun is used as a combiner, combining the two 25 ohm inputs into one 50 ohm output).
In the combiner configuration, the unbalanced side of the Balun assembly (the two 25 ohm ports), a coaxial cable ground shield connects one of the 25 ohm ports, by solder joint, to an RF launch area for RF signals. Then that ground shield traverses along a curved ground circuit path on a circuit board to the balanced side, and is connected to ground at the balanced side. That ground shield is continuously soldered to the circuit board trace. Further, a signal conductor of the coaxial cable projects outwardly from the unbalanced side to the second 25 ohm port and joins, by a solder joint, with an RF launch area for RF signals. The same, second 25 ohm RF launch area is connected with a second RF trace of equal electrical length, as the coaxial shield trace and is routed separately to a common, i.e. shared, balanced side ground. The signal conductor of the remainder of the coaxial cable joins, by a solder joint, with the balanced RF launch area for RF signals. The RF launch area is defined by a 50 ohm microstrip circuit trace on the circuit board at the RF output.
The known Balun assembly is provided on both sides of a known bidirectional, push-pull circuit. Both sides of the known push-pull circuit have respective RF inputs, and both sides are coupled by a respective Balun assembly to a single RF output. For example, the respective Balun assembly extracts unbalanced RF input signals from the two 25 ohm inputs characteristic impedance, and provides an RF output signal of 50 Ohms characteristic impedance at the launch area at the RF output. The balanced side and the unbalanced side of the respective Balun assembly have different characteristic impedances. Further, the unbalanced side outer conductor and signal conductor are intended to have RF signals 180 degrees out of phase, due to the conservation of charge. Adjusting their physical dimensions, for example, their dimensional lengths, assures proper match at respective RF signals.
The respective Balun assembly comprises a balanced side having a microstrip transmission line of known construction. A portion of the microstrip transmission line comprises a curved circuit path on a surface of a circuit board of known construction. An unbalanced side of the respective Balun assembly is constructed, in part, as having a coaxial cable, for example, of 50 Ohms characteristic impedance. A shield conductor of the coaxial cable is colinear with a curved ground circuit path on the same surface of the circuit board.
On the circuit board, the curved circuit paths of the balanced and unbalanced sides of a respective Balun assembly are curve back upon themselves, along their lengths to attain compactness in size. Further, the curved circuit paths of the respective Balun assembly are symmetrical. Therein lies a problem. The curved circuit paths are shaped primarily to attain their symmetry and compactness in size. However, the curved ground circuit path constrains the coaxial cable to extend along the curved circuit path, and the exposed signal conductor of the coaxial cable is exposed at length from the shield conductor, which contributes to undesired impedance mismatch. Further, the exposed signal conductor must be shaped by bending to fit with the geometry of the Balun assembly on the circuit board. For example, the exposed signal conductor is shaped by bending to extend toward, and to join with, the RF launch area.
Because the exposed signal conductor must be shaped to fit the geometry of the Balun assembly, and be bent and joined to a perpendicular RF launch. A sharp bend of less than minimum radii is normally incorporated. The expanding dielectric over temperature cycles pushes on the exposed bent section, which contributes unduly to stress concentration of the signal conductor at the Balun assembly. Stress concentration in the signal conductor leads to undesired fracture of a solder joint between the signal conductor and the RF launch area, as the Balun assembly is exposed to changing environmental temperatures over the passage of time.
Further, because the exposed signal conductor must be shaped to fit the geometry of the Balun assembly, a bent and lengthy signal conductor results, which contributes unduly to impedance mismatch. For example, the exposed signal conductor contributes to impedance mismatch, for which impedance compensation is required to rectify undesired voltage standard wave reflection (xe2x80x9cVSWRxe2x80x9d) due to the severity of the impedance mismatch. The exposed length and unevenness of bend of the exposed signal conductor contributes to the severity of impedance mismatch, and increases the difficulty in providing the required impedance compensation.
Prior to the invention, fabrication of the known Balun assembly required shaping of the coaxial cable to fit with undesirable, predetermined locations of both the ground circuit path and the RF launch area on the circuit board, which contributed undue mechanical stress and undesired impedance mismatch at a junction of the coaxial cable and the RF launch area.
According to the invention, a Balun assembly has a coaxial cable that has a precise electrical length, an exposed signal conductor of the coaxial cable is of minimum length and of smooth and even curvature, by way of substantially straight portions of minimum length adjoining a portion of substantially smooth and even curvature and of minimum length, the signal conductor distributes thermal expansion and contraction thereof substantially throughout the portions of minimum length and the portion of substantially smooth and even curvature, and the signal conductor of minimum length and of smooth and even curvature lessens impedance mismatch.
An advantage of the invention resides in providing a Balun assembly of improved reliability and construction, and lessened VSWR.
A further advantage of the invention resides in a method of making a Balun assembly having an exposed signal conductor that contributes to lessened impedance mismatch, and distributes thermal expansion and contraction therealong to reduce stress concentration.
Further, according to the invention, a method of making a Balun assembly comprises the steps of, sizing a coaxial cable to a precise electrical length, shaping an exposed signal conductor that projects from the coaxial cable with a portion of substantially smooth and even curvature and of minimum length, to project an end of the signal conductor substantially straight toward an RF launch area defined by a microstrip area on a circuit board, and positioning the RF launch area on the circuit board so as to be directly opposite the remainder of the cable and in close proximity thereto to minimize the length of the exposed signal conductor, thereby contributing to lessened impedance mismatch, and thereby distributing thermal expansion and contraction along the portion of substantially smooth and even curvature to reduce strain on the joint.
A further advantage of the invention resides in a method of making a Balun assembly having an exposed signal conductor that contributes to lessened impedance mismatch, and that distributes thermal expansion and contraction therealong to reduce stress concentration.